1. Field of the Invention
The present invention relates to double beam optical null spectrophotometers and, more prticularly, to improved apparatus for calibrating such spectrophotometers which corrects photometric errors introduced by radiation source inhomogeity and other imperfections in the optical system of the spectrophotometer. 2. Description of the Prior Art
In spectrophotometers of the above type, radiation from a source is switched between sample and reference beam paths and is recombined at a detector. A sample to be analyzed is positioned in one beam path and the resulting error signal at the detector is coupled in a feedback loop to adjust the position of an optical attenuator in the other beam path to balance the energy in the two paths and produce a null at the detector. When the system is at null, the position of the optical attenuator theoretically corresponds to the true transmittance of the sample. The slider of an attenuator transmitting potentiometer is driven in synchronism with the optical attenuator to generate a potentiometer output signal indicative of the optical attenuator position. The output signal is supplied to a recorder to provide a record of sample transmittance, usually over a range of wavelengths as scanned by a monochromator.
Unfortunately, however, because of radiation source inhomogeniety, attenuator imperfections, detector irregularities and the like, the position of the optical attenuator, and thus the transmitting potentiometer output signal, is not a true indication of sample transmittance over the range of optical attenuator positions from zero to 100 percent transmittance. As a result, a plot of the potentiometer output signal vs. true sample transmittance, which ideally should be linear, is nonlinear over at least a portion of the range. Unless corrected such nonlinearity can introduce an instrument error of 5 percent or more in the measurement of sample transmittance.
To linearize the relationship between the potentiometer output signal and sample transmittance, it has been the practice to hold the optical attenuator and optics to extremely close tolerances. Unfortunately, such an approach has not resulted in a truly linear relationship and has proven very expensive to implement.
To allow a certain amount of relaxation in the previously rigid manufacturing tolerance requirements, U.S. Pat. No. 3,504,977 (Matthews), assigned to the assignee of the present invention, describes a calibration method and apparatus incorporating an attenuator transmitting potentiometer having multiple taps and trimmer potentiometers connected to the taps. The optical attenuator is progressively positioned in the beam path and, for each position, the trimmer potentiometers of the transmitting potentiometer are adjusted to tailor the actual potential at each position along the transmitting potentiometer to a value representing true sample transmittance. While this approach does correct for instrument nonlinearity, the multiple tap trimmer potentiometer arrangement is complex and expensive and is thus restricted in commercial implementation. As a result, a need exists for a simple and inexpensive apparatus for calibrating a double beam optical null spectrophotometer to generate and record a signal truly indicative of the transmittance of a sample to be measured. The present invention fulfills these needs.